What’s a few degrees between species?
Even if they accept that anthropogenic global warming is occurring, some might wonder “why should I care?” This section will review what scientists expect to happen as the result of a warming world.
“What does this have to do with peace?”
In 2007, the IPCC shared the Nobel Peace Prize with Al Gore. Many asked the question, “What does climate change have to do with peace?”
The Pentagon, an independent group of retired admirals and generals, and the US Intelligence Community have issued reports on the dangers of climate change and its impact on world stability. The Pentagon report focused on a “what if” scenario of the unlikely event of the shutdown of the ocean conveyer belt, which would cause significant regional climate change (see next section for more on this tipping point). The generals and admirals focused on expected climate change and the consequences for international relations. The Intelligence Community addressed the effects of climate change up to the year 2030. The National Intelligence Assessment (NIA) that they authored is still classified, but the key findings were presented in a public hearing:
. . . the impacts will worsen existing problems, such as poverty, social tensions, environmental degradation, ineffectual leadership, and weak political institutions. Climate change could threaten domestic stability in some states, potentially contributing to intra- or, less likely, interstate conflict, particularly over access to increasingly scarce water resources. We judge that economic migrants will perceive additional reasons to migrate because of harsh climates, both within nations and from disadvantaged to richer countries.
The NIA concluded that, in the short term, the biggest risk to US national security would be from the destabilization of developing regions. Poor nations are the most vulnerable to climate change. The Network of African Science Academies agrees:
. . . we firmly believe that [the challenges of energy, climate, and sustainability] are even more daunting for the most impoverished, science-poor regions of the developing world, especially in Africa. These poor regions not only lack the resources to cope and adapt, but they also do not have adequate capacity in science and technology to successfully address the challenges of the future.
The conflict in Darfur is the classic example. The existing civil unrest in Darfur is amplified by increasing desertification. The Sudanese government has been either unable or unwilling to institute policies to make better use of dwindling resources. The result is conflict between local farmers and nomadic grazers, an uprising against the government, and general misery for inhabitants of the region.
In this example from India, the dark areas on the map rank as the most vulnerable to climate change, with the crosshatched areas showing competitive pressure on local agriculture from globalization.
There are 1 billion people living in India today and it wouldn’t take much to push Indian society into a climate induced crisis. Special susceptibility to climate change is common throughout the developing world, so the belief by some skeptics that anthropogenic global warming is merely a plot to bring misery to developing countries could not be further from the truth.
“It’s only a few degrees”
Why would a few degrees cause so many problems? Temperatures can fluctuate many times that amount over the course of days and weeks, to say nothing about the difference between the day and night. But an increase of a few degrees in the average global surface temperature anomaly is a substantial difference. Over the coming century, we will see temperature increases anywhere from 1 ° under the very best circumstances to 4 ° or more under the worst. Recall that the difference between today and the depth of an ice age is just 4-6 ° (see section 7).
To better illustrate potential changes, a bell curve is used. 
The top of the curve indicates normal temperatures, or the temperatures that occur with the highest frequencies. A shift to the warm side increases the likelihood of warm weather and decreases the likelihood of cold weather.
In some cases, there may be an increase in variance, which is another way of saying an increase in the temperature extremes.
That is, there is an increase in the frequency of cold temperatures, and an increase in frequency of warm temperatures. The extremes gain at the expense of normal temperatures, and the bell curve is squashed.
A combination of these two effects is likely, and the result would be only a slight decrease in the frequency of cold weather. However, we would see a huge increase in the frequency of warm and very hot weather.
What this means for everyday life can be described with this example from Europe. In the summer of 2003, there was an historic heat wave that occurred primarily in France.
From history, we can see how the frequency of warm and cold weather follows the bell curve. But the temperatures of 2003 were well outside the norm (pane b) and are believed to be unprecedented in at least 600 years. When the climate between 1961 and 1990 is simulated, a frequency plot much like observations is generated (pane c). But when the end of the 21st Century is simulated using one of the IPCC’s emission scenarios, the extreme temperatures of 2003 would fall right in the middle of a much broader and warmer bell curve (pane d).
What we consider extreme today would be the new normal. 30,000 people died over just a few days in 2003 so this is quite significant. Most of those deaths could have been prevented with the right preparation, but the climate and thus the environment would be radically transformed under this new climate. The heat wave led to large wildfires due to dry conditions, reduced river flow which interfered with inland navigation, and extreme glacial melt in the Alps. The heat wave also caused large reductions in agriculture, with uninsured losses in France alone worth €4 billion:
And this is just the “normal” temperatures we can expect. Now consider the new definition of “hot.”
In addition to an increase in temperature extremes, we should expect an increase in precipitation extremes as well. As described in the previous section, we have already measured this and it will only increase in the future. Some areas will see increased drought, and others will see increased flooding. Some may even see both as weather extremes are exaggerated at different times of the year.
With more moisture held by the atmosphere overall, precipitation will come in shorter bursts, which is shown here as the precipitation intensity.
The dark purple areas show where this is expected to be most significant, which is primarily in higher latitudes.
The increase in precipitation intensity comes at the expense of other times of the year.
Most of the world will go for longer periods of time without significant precipitation.
These maps represent the output of multiple computer models from different teams of scientists. As with all model scenarios, the specifics must be taken with a grain of salt, but the general conclusions are likely to be accurate.
The importance of glaciers and snowpack
Of particular importance to the water supply of a billion people (including the western United States) is the fate of glaciers and snowpack. Accelerated melting of glaciers leads to increased flooding and erosion in the short term, followed by large declines as they are exhausted. Winter snowfall is no longer sufficient to replace what melted the previous summer and the glaciers dwindle to nothing. Accelerated melting of regional snowpack results in faster runoff in the spring, leading to shortages by the end of the summer. In addition, reduced runoff will disrupt electricity generation in regions that depend on hydro-electric power.
Balmy green paradise?
Skeptics will often tell you that warmer temperatures and additional CO2 are good for us, because both work to enhance plant growth. In general, warmer temperatures bring longer growing seasons, and CO2 is essential for all plant life. In practice, crop yields are limited by things other than CO2, for example, water and nitrogen. In addition, extreme weather, such as periods of intense rain, intense draught, or heat waves disrupts agriculture at various stages of a crop’s lifecycle and yields suffer as a result.
Under most scenarios, there will be “winners and losers” since different regions will see different impacts. The following graphs summarize 69 different studies on crop yields. The results are highly uncertain due to the number of variables that affect agriculture. However, the general conclusion is that agriculture in mid to high latitudes (such as the US) see an improvement with local temperature increases between 1 and 3 °, but tropical areas see reductions. This is only true in the broadest sense, and the specifics of each area will dictate whether or not local agriculture improves or declines.
For corn, real world yields in the mid to high latitudes show an increase in productivity but this benefit is reduced as local temperatures continue to rise.
The orange line shows yields under existing agricultural practices, while the green line shows yields if we optimize agriculture to the new climate. In the tropics, yields can be maintained with adaptation , but drop off if existing methods are used.
For wheat, yields are greatly increased with warming. But for the tropics, yields with no adaptation decrease sharply.
All of this assumes that there is sufficient water for these crops. If available water is reduced in specific areas, agriculture will suffer and whatever marginal gains will be lost.
With warming temperatures, there are both positive and negative health impacts, but the negatives far outweigh the positives. 
Sea level rise: adding up millimeters
Sea Level rise is another concern for the future, which may seem odd given that it is measured in millimeters per year. The current rate of sea level rise is over 3 mm per year, which over the course of a century would add more than a foot. This is twice the rate of the last century, and more than 4 times the rate of the early portion of the 20th Century.
Rising sea levels threaten infrastructure both directly through inundation and indirectly by extending storm surge and increasing erosion. Sea level rise causes salt water to encroach on drinking water and estuaries.
This map shows highly populated river deltas that are vulnerable to just the current rate of sea level rise, to say nothing of accelerated rise. 
The majority of these areas are in developing countries, although not surprisingly, the Mississippi delta region is also considered vulnerable.
“The IPCC cut its sea level rise figures in half!”
When the Fourth Assessment Report (AR4) Working Group 1 (WGI) Summary for Policymakers (SPM) was released in February 2007, there was much talk of the IPCC cutting their sea level rise figures in half. The Third Assessment Report (TAR) from 2001 gave an upper range of 0.88 m by 2100, which is about 2 feet 11 inches.
The AR4 SPM gave an upper range of 0.59 m, but this excluded “rapid dynamic changes in ice flow,” as they indicated. 
In the body of the report, this is shown as “scaled up ice sheet discharge” which equals 0.17 m in the worst case scenario.
This is accelerated ice loss from Greenland and West Antarctica, and was included in the TAR’s figures. These numbers total 0.76 m, which is clearly not half of 0.88 m. There are also some other minor differences, but the point is that the TAR and AR4 numbers cannot be directly compared.
But 0.76 is still less than 0.88, so why are the numbers decreasing? The numbers that receive the most attention are the upper range figures. There are lower range figures as well, and those numbers are increasing. In other words, the confidence is increasing as the lower and upper bounds move closer together. But even that isn’t the whole story.
Accelerated sea level rise
Sea level rise of 1 to 3 feet over a century is one thing, but headline grabbing images of Florida under water requires multiple meters. The first thing to be aware of is that the IPCC scenarios end with the 21st Century, and the world will obviously continue on after 2100.
There are basically three parts to sea level rise, two of which are better understood, and for the third we are basically in the dark. Just over half of current rise is the result of the additional space taken up by warmer waters ("thermal expansion"). The next portion is due to the melting of ice sheets and mountain glaciers. These two components make up the 0.59 meters of sea level rise.
The final component is the big unknown. This is the stability of the ice sheets, namely Greenland and West Antarctica. If we were just talking about melting ice, it would take thousands of years for them to disappear. If that was the case, civilization would have enough time to adapt. But ice doesn’t just melt. It flows, and flowing ice does not behave in a linear fashion. This is the portion that was labeled “scaled up ice sheet discharge” in AR4’s table. We currently have no computer models capable of accurately modeling the flow of ice, so the 0.17 m figure is little more than a guess.
As the surface of ice sheets melt, the ice goes from white and snowy to dark and glossy. The darker color lowers the albedo which accelerates the warming. Pools of water form on the surface and tunnel down to the bedrock. 
Not only does this weaken the sheet, but it lubricates the bottom of the ice sheet which causes it to accelerate as it slides into the ocean.
The problem is amplified by retreating glaciers that flow from the interior of Greenland into the ocean. Much of Greenland itself is below sea level. The land has been depressed due to the immense weight of the ice sheets (blue areas are below sea level). 
As glaciers retreat, the ocean follows them into the interior. In the case of a few glaciers (such as those indicated) a point may be reached where the freshly exposed ground is below sea level, and the ocean rushes in, further eating away at the base of the ice sheet. The problem is made worse by rising sea levels.
Warming water also undercuts the floating ice shelves of Antarctica, which further weakens them. Although disintegrating ice shelves do not directly contribute to sea level rise, they often act to buttress the ice on land. When the shelves are removed, the land ice is no longer held back, and it rushes to the sea.
Our best evidence of the speed of ice sheet disintegration comes from studying the past. During the peak of the last interglacial period, temperatures were about 2 ° warmer and sea levels were 4-6 meters higher. Evidence suggests that sea levels during this period rose at an average rate of 1.6 meters per century over several centuries. Only the most optimistic future scenarios show total warming less than 2 ° by the end of the 21st century. In the near future, we will likely recreate the conditions of the last interglacial period, and the major question is how quickly it will take for the ice sheets to respond.
“Polar bears are thriving!”
The melting of the Arctic has significant implications for the polar bear, which has become an icon of global warming. This has drawn the scrutiny of skeptics who declare that polar bears are doing great so there is no need to worry. The reason that polar bears have increased in numbers over the past 3 decades is due to the International Polar Bear Agreement, signed in 1973 by Canada, Denmark (which governs Greenland), Norway, the Soviet Union and the United States. All non-traditional hunting of polar bears was banned, and as a result, most populations recovered. Until recently, the direct effect of man outweighed climate change.
The problem is that polar bears depend on sea ice for the hunting of seals and other large marine mammals. Polar bears require the high fat content of these animals to survive, and when forced onto land for long periods of time the bears have no choice but to fast. As more summer sea ice melts, they are cut off from their hunting grounds and they starve. We have already observed this effect in the Western Hudson Bay and Southern Beaufort Sea (north of Alaska and northwest Canada). Based on scenarios that have already been shown to be optimistic (see next section), the polar bear will be committed to extinction by the end of this century, if not much sooner.
Ocean acidification and coral bleaching
One impact that is often lost in the debate about climate change is the ongoing acidification of the ocean. As shown in the previous section, the oceans are absorbing large amounts of anthropogenic CO2. This decreases the pH of the oceans, making them more acidic. More accurately, it makes them less alkaline or “less basic”. This has serious consequences for shell forming organisms and thus the entire ocean food chain. 
As the pH falls, it becomes increasingly difficult for the organisms to form their shells. Below shows the effect on a sea butterfly subjected to conditions of the Southern Ocean expected by 2100.
Coral reefs are also susceptible, and they are already under stress due to continued bleaching caused by warming waters (see previous section). When water temperatures reach about 1 ° above the normal maximum for a sustained period, the symbiotic algae that live within the reefs die, and the reefs turns white. 
When the waters warm 2 ° the reefs themselves die. The combination of coral bleaching and ocean acidification will likely cause the destruction of coral reefs throughout the world. Many marine organisms depend on coral reefs for their habitat and food supply.
Humans depend on the reefs as well, both for fishing and protection against storm surge. Many Pacific islands are both protected and supported by coral reefs, so as they decay, the islands literally sink. Increased sea level, increased storm surge, and sinking islands will make many island nations uninhabitable.
Nature vs. climate change
Biologists take the scenarios generated by the climate scientists and then apply them to our understanding of the living world. The following is a summary of some of the more significant findings within AR4. The temperature increase is in relation to current temperatures.
 (Schwartz & Randall, 2003) Online here.
 (Sullivan, et al., 2007) Online here.
 (Fingar, 2008) Online here.
 (NASAC, 2007) Online here.
 (Adger, et al., 2007) Online here. Figure 17.2.
 (Folland, et al., 2001) Online here. Figure 2.32.
 (Alcamo, et al., 2007) Online here. Figure 12.4.
 (Chuine, et al., 2004) Online here.
 (Easterling, et al., 2007) Online here. Box 5.1.
 (Meehl, et al., 2007) Online here. Figure 10.18.
 (Easterling, et al., 2007) Online here. Box 5.1.
 (Confalonieri, et al., 2007) Online here. Figure 8.3.
 (Nicholls, et al., 2007) Online here. Figure 6.6.
 (IPCC, 2007) Online here. Table SPM.3.
 (Meehl, et al., 2007) Online here. Table 10.7
 (Zwally, Abdalati, Herring, Larson, Saba, & Steffen, 2002) Online here. (free registration required)
 (Rhines, Brazelton, & Lindahl, 2004) Online here.
 (Rohling, et al., 2008) Online here.
 (Holland, 2007) Online here.
 (StormCenter Communications, 2006) Online here.
 (Fischlin, et al., 2007) Online here. Adapted from figure 4.4.
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